IR transmitting optical fiber

ABSTRACT

The present invention is directed to a cladded optical fiber and a process for manufacturing the same. The cladding and core are halide materials. An interface for inhibiting radiation scatter is provided at the boundary between the halide cladding and the halide core. The process steps include extruding a first halide or halide core from a first chamber, and extruding a second halide or halide cladding from a second chamber into contact with the halide core. The halide cladding is joined to the halide core at the boundary.

This is a division application of pending prior application Ser. No.134,276 filed Dec. 17, 1987.

SCOPE OF THE INVENTION

The present invention is directed to a cladded optical fiber fortransmission of electromagnetic energy in the infrared region and aco-axial extrusion process for fabricating such fibers.

BACKGROUND OF THE INVENTION

There is a need for an optical fiber, having a halide core with a halidecladding, for the transmission of infrared electromagnetic radiation,which is capable of economical mass production, yet minimizes radiationtransmission losses. Transmission losses arise from impurities in thehalide starting materials, from impurities in the halide materialsintroduced from the extruder and the extruder die, from irregularsurface boundary conditions at the core-cladding interface, fromnon-uniform mixing of the core and the cladding materials at theinterface, and from the presence of grain boundaries between thenumerous crystals forming the core. Each of these problems decreases thetransmission efficiency of the core material by increasing the scatterof the transmitted electromagnetic radiation.

U.S. Pat. No. 4,253,731 discloses a metal halide (AgBr) fiber (or core)which is clad with another metal halide (AgCl). The silver bromide corehas a fine-grained crystalline structure which is produced by anextrusion process, preferably in the low temperature range set forth inFIG. 3 of the patent. In the extrusion process, a coaxial or compositebillet, i.e. a sleeve of cladding surrounding a cylindrical core, isheated in a single chamber and extruded through a diamond die accordingto FIG. 2 of Pat. No. 4,253,731.

Chen, D. et al, "Fabrication of Silver Halide Fiber by Extrusion," FiberOptics: Advances in Research and Development, ed. by B. Bendow et al,Plenum, N.Y., p. 119-122, 1977, also, discloses a clad fiber similar tothe one described in U.S. Pat. No. 4,253,731

Japanese Patent Application No. 1980-87508, published in the JapanPatent Journal No. 1982-13410, discloses two embodiments of an infraredoptical fiber and methods of producing each. In the first embodiment, acladded optical fiber is produced by a first extrusion step from acomposite billet. The resulting cladded fiber is then subjected to asecond extrusion step during which a plastic coating, e.g. polyethylene,polypropylene, nylon-6, polyacetal or acrylic, is applied on the claddedfiber.

According to the second embodiment of Japanese Patent Application1980-87508, an uncladded optical fiber having a plastic coating isproduced in a two-step extrusion process. An uncladded metal halidebillet, i.e. not a composite billet, is extruded into a fiber during afirst step, and a plastic coating is then extruded around the fiber in asecond step.

U.S. Pat. No. 4,678,274, assigned to the assignee of the hereof patentapplication, discloses a cladded optical fiber, having a halide core andhalide cladding extruded from a composite billet having a covering of apolymer film.

Pinnow, D. A. et al, "Polycrystalline Fiber Optic Waveguides ForInfrared Transmission", Applied Physics Letters, Vol. 33, No. 1, Jul. 1,1987, disclose optical fibers formed by extrusion of thallium bromide orthallium bromoiodide. The extruded fibers are then sheathed in aloose-fitting polymer sleeve. The core fibers are polycrystalline.

Japanese Patent Application No. 1981-140929, published in Japan PatentJournal No. 1983-43404, discloses an improved extrusion die forproducing optical fibers from halide billets. Contamination of thebillet from chamber wall impurities is reportedly avoided by a die whichhas a diameter less than the diameter of the billet. The die has abeveled attack surface which meets the billet. The die, in effect,shears off an annular portion of the billet which contains thecontaminants.

Vasil'ev, A. V. et al, "Single-Crystal Fiber Waveguides For The MiddleInfrared Range," Sov. J. Quantum Electron., Vol. 11, No. 6, Jun., 1981,disclose a single-crystal halide optical fiber which is grown incapillaries. The single-crystal fibers do not have a cladding.

U.S. Pat. No. 4,583,821 discloses a cladded optical fiber, formed frommixed AgBr/AgCl crystals surrounded by a protective layer. The cladfiber is extruded from a composite billet as discussed in U.S. Pat. No.4,253,731. An extrusion temperature between 100° C. and 380° C. isreported.

U.S. Pat. No. 4,552,434 discloses an optical fiber having a halide coreand a halide cladding (See FIG. 9a) which is produced by placing a corebillet into a sleeve of cladding and drawing the sleeve into contactwith the core billet. A gap of about 0.01-0.1 mm is maintained betweenthe billet core and the sleeve cladding before the drawing step. Thefollowing drawing temperatures are disclosed for the halide material ofPat. No. 4,552,434: 120° C.-358° C. for KRS-5; 100°-370° C. for silverchloride; 180°-370° C. for cesium iodide (melting point 626° C.).

Mimura, Y. et al, "CsBr Crystalline Fiber For Visible and InfraredTransmission," Japanese Journal of Applied Physics, Vol. 20, No. 1, p.L17-L18 (January 1981), disclose a cesium bromide optical fiber which isinserted into a Teflon® jacket.

Japanese Public Patent Disclosure Bulletin No. 56-104302 discloses anoptical fiber with a halide core and a halide cladding. This fiber isproduced by forming a composite billet, placing the billet into a sealedmetal pipe, and drawing down the pipe's diameter, i.e. cold working,until the desired fiber diameter is reached.

Harrington, J., "Crystalline Infrared Fibers," Proc. Soc. Photo-Opt.Inst. Eng., Vol. 226, February 1981, and Harrington, J. et al.,"Scattering Losses in Single and Polycrystalline Materials for IR FiberApplications," Adv. in Ceramics, vol. 2, pp. 94-103 (1981) studied thescattering losses in single crystal and polycrystalline KCl and KRS-5.The authors report that polycrystalline materials scatter more radiationthan single crystal materials.

Sakuragi, S. et al, "IR Transmission Capabilities of Thallium Halide andSilver Halide Optical Fibers," Adv. In Ceramics, Vol. 2, pp. 84-93,1981, describe experiments with unclad halide fibers.

SUMMARY OF THE INVENTION

The present invention is directed to a process for manufacturing acladded optical fiber. The cladding and core are halide materials. Aninterface for inhibiting radiation scatter is provided at the boundarybetween the halide cladding and the halide core. The process stepsinclude extruding a first halide or halide core from a first chamber,and extruding a second halide or halide cladding from a second chamberinto contact with the halide core. The halide cladding is joined to thehalide core at the boundary.

DESCRIPTION OF THE DRAWING

For the purpose of illustrating the invention, there is shown in thedrawings a form which is presently preferred; it being understood,however, that this invention is not limited to the precise arrangementsand instrumentalities shown.

FIG. 1 is sectional schematic view of the extrusion apparatus andillustrates the co-axial die assembly and a direct extrusion plunger.

FIG. 2 is a sectional schematic view of the extrusion apparatus andillustrates the co-axial die assembly and a hydrostatic extrusionplunger.

FIG. 3 is an enlarged schematic view of the coaxial die assembly.

FIG. 4 is a graph illustrating the relationship between crystal grainsize of the fiber core and extrusion temperature (normalized to halidemelting point, 1.00 being the melting point of the material).

FIG. 5 shows a cladded optical fiber of single-crystal KBr core and apolycrystalline KCl cladding at a magnification of 250 times which ismade in accordance with the present invention.

FIG. 6, on the left, shows the cladded fiber of FIG. 5 at amagnification of 300 times and, on the right, shows the blocked portionof the left side photograph at a magnification of 1500 times.

FIG. 7 shows a prior art cladded fiber at a magnification of 100 times(polycrystalline KBr core and polycrystalline KCl cladding) made from acomposite billet, as described in U.S. Pat. No. 4,678,274.

FIG. 8 shows a prior art cladded fiber at a magnification of 80 times.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings, wherein like numerals indicate like elements,there is shown in the drawings part of an extrusion apparatus 10.Extrusion apparatus 10 is merely exemplary of any typical extrusionapparatus and is not limiting on the present invention as will beapparent from the discussion below. "Extruding", as used herein, refersto any process or apparatus which delivers material to an extruding die,i.e. plungers, pumps, screw feeders, etc.

The process, described hereinafter, is an extrusion process used to makean optical fiber having a substantially single crystal halide core witha polycrystalline halide clad joined thereto. The optical fiber isdescribed in greater detail below. The process utilizes separateextrusion chambers for receipt of the individual halide billets for thecore and clad. Both billets are simultaneously heated to a temperatureclose to, but below, the halides' melting points, and then rammedthrough a co-axial die. The co-axial die, in communication with bothchambers, forms the core and applies the clad thereto in a manner whichsubstantially eliminates core and clad material mixing at the core-cladinterface, yet joins the core to the clad at the interface and forms asmooth interface. The resulting cladded halide optical fiber transmitsIR radiation by providing a substantially single crystal core and byminimizing leakage losses at the interface.

Apparatus 10 includes a body 12 having a first chamber 14 and a secondchamber 16. Both chambers 14 and 16 may have a diameter of between about0.1-0.6 inches (2-15mm) and a length of between about 2 to 10 inches(50-250 mm) and are sized to receive a billet, e.g. a solid cylinder orrod. However, these dimensions are merely exemplary. Other dimensionsreadily apparent to those skilled in the art are possible. Both chambers14 and 16 are in communication with a die assembly 28.

Body 12 is surrounded by a plurality of heating elements 24. Heatingelements 24 are conventional and provided with sufficient heatingcapacity to heat the extrusion apparatus, the halide materials and thedies to a temperature high enough such that the halide materials areductile, but below the melting temperature of the halide material.

The extrusion apparatus 10 should also be provided with an inert gaspurge system which allows the halide starting materials and resultingclad fiber to be surrounded with a gas, such as argon, helium, etc. Thegas displaces any contaminating gas (moist air, etc.) and significantlydecreases oxidation, corrosion and other reactions between the toolingmaterials and the halides. The gas, thereby, improves the quality of thefinished product.

First and second plungers are provided for insertion into theirrespective chambers. The plungers are driven by an actuator (not shown)which is conventional. The plungers may be either a direct plunger asillustrated by plunger 18 in FIG. 1, or a hydrostatic plunger asillustrated by plunger 18' in FIG. 2. Either type of plunger can be usedin either chamber. The direct plunger 18 has a diameter which extendsacross the entire internal diameter of its chamber. The hydrostaticplunger 18' has a diameter which is less than the internal diameter ofthe chamber. Moreover, the plungers essentially create a fluid pressure,thus it should be understood that a high pressure, halide or othermaterial, pumping system that effects the appropriate fluid typepressure could also be considered within the scope of this invention.

The hydrostatic plunger 18' has a number of advantages over the directplunger 18. The hydrostatic plunger permits a more uniform exertion offorce on the material within the chamber. It reduces the extrusion forceby reducing friction between the plunger and the chamber wall andcreates a buffer from the potentially contaminating chamber wall. It maybe readily appreciated that the hydrostatic plunger uses the ductilematerial within the chamber as a lubricating fluid to reduce wallfriction. The ductile material in the buffer zone prevents migration ofcontaminants from the chamber wall into the central portion of theductile material which is extruded.

A coaxial extrusion die is shown as die assembly 28, and comprises anupper die 30, a lower die 36 and a holder 42. See FIG. 3. Upper die 30is in communication with first chamber 14 and is provided with a halideflow passage 46 defined by a fiber-forming surface 32. The passage 46 isconfigured to reduce the cross-sectional area of the chamber to thediameter of the fiber core. A cladding-forming surface 34 is provided onupper die 30 and is coaxial with the halide flow passage. The lower die36 is also provided with a through passage which terminates into outlet50 and includes a clad-forming surface 38. The passage through the lowerdie 36 is aligned with and downstream from the first-mentioned flowpassage 46. The passage through lower die 36, at its narrowest point,has a diameter equivalent to the diameter of the clad fiber (i.e. thecore plus the clad). Both upper die 30 and lower die 36 are surroundedby a holder 42 which includes a flow passage 48 therethrough. Flowpassage 48 is in communication with second chamber 16.

Upper die 30, lower die 36, and holder 42 define an annular channel 44through which cladding material is allowed to surround the fiber-formingpassage through upper die 30. Alternately, holder 42 can be made part ofdies 30 and 36 or part of one or the other die. Channel 44 is incommunication with flow passage 48. Clad forming-surface 34 of upper die30 and clad-forming surface 38 of lower die 36 define a clad-compressionchannel 40 which is co-axial with the fiber-forming passage 46 of upperdie 30. See FIG. 3. The above-described die configuration allowsuniformity of material velocity at a point where the core material andthe clad material meet.

At the beginning of operation, the core is preferably extruded slightlyahead of the cladding, however, the cladding could be extruded ahead ofthe core or both the core and cladding may be extruded together. Duringthe operation both core and cladding exit together and at the same rate.

The materials of construction for the plungers, body, and co-axialextrusion die assembly are chosen to have mechanical strength andresistance to corrosion at the required operating temperatures which areclose to, but just below, the halides' melting points. For example, whendealing with softer, lower melting point material, such as the thalleoushalides (e.g., KRS-5 and KRS-6), materials such as steel, chrome-coatedsteel, nickel or TIC (titanium carbide), are satisfactory. The extrusiontemperatures of those halides are sufficiently low to permit the use ofsofter tooling materials which provide sufficient strength and corrosionresistance.

Other halides, such as NaBr or KCl, are harder and have significantlyhigher melting temperatures than KRS-5 and KRS-6. Extrusion of thesehigher-melting materials requires different tooling materials to providethe necessary strength and corrosion resistance. For example, materialssuch as hardened high nickel, high chrome alloys, ceramics or other veryhigh temperature metal or ceramic alloys, alone or with coating orsurface modifications are required.

When very corrosive halides are being extruded, or when extrusion isthrough uncoated tooling, the risk of contamination of the fiber-formingmaterials may be reduced by hydrostatic extrusion.

The selection of materials for the dies is also of importance andfollows from the above discussion. The fiber-forming surface 32, andclad forming surfaces 34 and 38, should be finished to a surfaceroughness of less than approximately 10 micro-inches, and preferablyless than 5 micro-inches. These surfaces may be diamond, but this maynot be economically feasible. Moreover, diamonds may not be suitable forextrusion at very high temperatures because of diamond degradation. Thepreferred materials of construction for the die are solid ceramics, suchas aluminum oxide-titanium carbide and Sialon (i.e. Si-Al-O-N). Withregard to Sialon, see "Ceramics Fire the Imagination", MaterialsEngineering, Jul., 1986, pp. 31-35. Aluminum oxide is also a highlynon-reactive material which is capable of producing a smoothoptically-clear halide fiber. Moreover, these relatively inexpensiveceramics can be fabricated into a wide variety of die geometries. Thesurface finish, roughness and coating, are applied along the entirehalide-contacting surfaces of the die, not just along the exit end ofthe die.

Dies formed from carbide or ceramic-coated metals may be used whenextruding lower-melting halides. However, use of these die materials isunsatisfactory when extruding high-melting halides.

The halide materials used in the above-described apparatus include metalhalides, alkali earth halides or alkali halides. Preferably, thesehalides are selected from group Ia, Ib and IIIa halides and mixturesthereof, for example,

Ia: NaCl, NaBr, NaI, KCl, KBr, CsI, CsBr;

Ib: AgBr, AgCl, AgI;

IIIa: TlBr, TlCl, TlI, thallium bromide-iodide or "KRS-5", thalliumbromide-chloride or "KRS6";

Mixed Halides: TlBr:TlI, AgBr:AgCl.

The extruded materials should be essentially contamination-free andeither crystalline or polycrystalline. Pure, doped, or mixed halides maybe selected to provide the desired index of refraction, strength orother pertinent characteristics as is known in the art. Sintered powdercompacts may not be suitable as extruded materials because of impurityabsorption on surfaces and potential incorporation of foreign particles.However, if these purification problems can be overcome, sintered powdercompacts are suitable for use in the invention.

Optical quality halide billets suitable for the use in the process ofthe invention are available from the following: Harshaw Chemical Co.,Solon, Ohio; Optovac Company, N. Brookfield, Mass.; E. Merck, Darmstat,W. Germany; Fluka Chemical Corp., Ronkonkoma, N.Y.; Heico Div.,Whittaker Corp., Delaware Water Gap, Pa. If higher purity billets areneeded, they may be grown from melt according to well-known purificationand growth processes, such as the Bridgeman or the Czochralski process.The Bridgeman growth process may be utilized to generate a single largecrystal to provide a highly purified single crystalline ingot.

The halide used to form the cladding layer should have an index ofrefraction lower than the core halide.

Clad fibers having substantially single crystal cores are produced fromthe process of the present invention when extrusion temperaturesapproaching the halide melting temperature are used. FIG. 4 illustratesthe dependency of core crystal size on extrusion temperature for NaCl,KBr and CsBr. It is apparent from FIG. 4 that crystal grain sizeincreases as the extrusion temperature approaches the melting point ofthe extruded materials. Therefore, it is preferred to extrude the halidematerials close to their respective melting temperatures as reasonablypossible.

The upper limit of the extrusion temperature is dictated, in one aspect,by the condition of the exiting fiber. For example, a temperature isreached beyond which the newly-extruded hot fiber cannot support its ownweight. Useable extrusion temperatures are further limited, in anotheraspect, by limitations in accurately monitoring and controlling theextrusion temperature.

The information presented in FIG. 4 is intended to be exemplary of therelationship between halide extrusion temperature (the extrusiontemperatures are normalized to the specific halide's melting point) andcrystal grain size. The same information is readily obtainable for otherhalides via experimentation in which the halide material is brought tovarious temperatures below its melting point and extruded. Thesolidified halides are then examined, in known ways, to determine thecrystal grain sizes. The curves of FIG. 4 are averages of severalextrusion velocities (approximately 8 inches per minute to about 80inches per minute) and several fiber diameters (about 0.017 inches to0.030 inches).

Fibers according to the present invention may be manufactured atvelocities as high as 5,000 mm per minute (approximately 200 inches perminute). Those skilled in the art will appreciate that this uppervelocity limit is dictated, in part, by the strength of the toolingmaterials.

The optical fiber produced by the instant process provides asubstantially single crystal core having a polycrystalline cladding witha relatively smooth clad-core interface which inhibits electromagneticradiation scatter. The core and clad are "joined" which means the coreis intimately and essentially coaxially in contact with the clad, yetthere is little-to-no mixing of the core and clad materials.

"Substantially single crystal" core means not only a core comprising asingle crystal, but also includes cores formed by a small number oflarge single crystals, which have the transmission properties which aredesired. It is the object of the invention to minimize the number ofseparate crystals comprising the fiber core, preferably to a singleunitary crystal. Sectioning the fibers of the invention laterallyreveals that any such lateral cross-section (i.e., perpendicular to thelongitudinal axis of the fiber or an axial face) intersects only one,two or generally not more than three crystals. Longitudinal sectioningmay reveal the presence of a somewhat greater number of crystals alongthe length of the fiber core.

Referring to FIGS. 5 and 6 (axial face illustrated), a clad fiberaccording to the invention has a single crystal KBr core and apolycrystalline KCl clad. Note that there are only few cleavage defectsat the core-clad boundary. Even at higher magnifications (FIG. 5), theboundary shows very few cleavage defects. FIGS. 7 and 8 illustratecore-clad fibers made according to the prior art technique of extrudinga composite billet of a core billet surrounded by a cladding sleeve.Note the jagged clad-core boundary and the numerous crystal boundarywithin the core material

The products made according to the present invention, represent asignificant step forward in the area of low transmission-loss claddedoptical fibers.

The invention will now be described in greater detail with reference tothe following non-limiting Example.

EXAMPLE 1

A cylindrical billet of KBr and a cylindrical billet of KCl, comprisingthe core and cladding materials, respectively, are extruded by anapparatus as described above. The temperature of both the core halideand cladding halide at extrusion is 710° C. The plunger forces are about900 pounds for the core plunger and about 1900 pounds for the cladplunger. Extrusion is carried out in an argon atmosphere at a fiberfabrication velocity of 48 inches/min. FIGS. 4 and 5 are representativeof the product.

EXAMPLE 2

A billet of AgBr and of AgCl, comprising the core and claddingmaterials, respectively, are extruded by an apparatus as describedabove. The temperature of both the core halide and cladding halide atextrusion is 300 degrees C. The plunger forces are about 1300 pounds forthe core plunger and about 2000 pounds for the clad plunger. Extrusionis carried out in an argon atmosphere at a fiber velocity of 48"/min.FIG. 5 is representative of the product.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof and,accordingly, reference should be made to the appended claims, ratherthan to the foregoing specification, as indicating the scope of theinvention.

We claim:
 1. An optical fiber comprising:(a) a halide core having asubstantially single crystal of said halide across an axial face of saidcore, said single crystal extending axially along said core; and (b) apolycrystalline halide cladding surrounding said core,said core and saidcladding having different indices of refraction from one another, saidcore being prepared by the extrusion of a first halide billet through afirst die after said first billet was heated to a temperature just belowits melting point, and said cladding being prepared by the extrusion ofa second halide billet through a second extrusion die after said secondbillet was heated to a temperature just below its melting point.
 2. Theoptic fiber according to claim 1 wherein said core is a single crystal.3. The optic fiber according to claim 1 wherein said core and saidcladding comprise at least one halide selected from the group consistingof metal halides, alkali earth halides, alkali halides or mixturesthereof.
 4. The optic fiber according to claim 1 wherein said core andsaid cladding comprise at least one halide selected from the groupconsisting of Group Ia halides, Group Ib halides, Group IIIa halides andmixtures thereof.
 5. The optic fiber according to claim 1 wherein saidcore and said cladding comprise at least one halide selected from thegroup consisting of NaCl, NaBr, NaI, KCl, KBr, CsI, CsBr, AgBr, AgCl,AgI, TlBr, TlCl, TlI, thallium bromide-iodide, thallium bromide-chlorideand mixtures thereof.
 6. An optical fiber according to claim 1 whereinsaid cladding is dimensioned such that the outside surface of said coreis essentially in continuous intimate contact with the inside surface ofsaid cladding.
 7. An optic fiber according to claim 6 wherein said coreand said cladding are prepared by being simultaneously extruded throughsaid first and said second die, respectively, at the same rate.
 8. Acladded fiber waveguide for transmission of infrared frequenciescomprising:(a) a core of a first halide compound having a first index ofrefraction, said core comprising a substantially single crystal of thefirst halide compound; and (b) a cladding around said core of a secondhalide compound having a second index of refraction less than said firstindex of refraction, said cladding comprising a substantiallypolycrystalline material of the second halide compound, said core beingprepared by the extrusion of said first halide compound through a firstdie after said first halide compound was heated to a temperature justbelow its melting point, and said cladding being prepared by theextrusion of said second halide compound through a second die after saidsecond halide compound was heated to a temperature just below itsmelting point.
 9. An optic fiber according to claim 8 wherein said firsthalide compound and said second halide compound comprise at least onehalide selected from the group consisting of Group Ia halides, Group Ibhalides, Group IIIa halides and mixtures thereof.
 10. The optic fiberaccording to claim 8 wherein said first halide compound and said secondhalide compound comprise at least one halide selected from the groupconsisting of NaCl, NaBr, NaI, KCl, KBr, CsI, CsBr, AgBr, AgCl, AgI,TlBr, TlCl, TlI, thallium bromide-iodide, thallium bromide-chloride andmixtures thereof.
 11. An optic fiber according to claim 8 wherein saidcore and said cladding are prepared by being simultaneously extrudedthrough said first and said second die, respectively, at the same rate.12. An optic fiber according to claim 11 wherein said core and saidcladding are prepared by being simultaneously extruded through saidfirst and said second die, respectively, at the same rate.